1. Field of the Invention
The present invention relates generally to an optical device having an optical waveguide structure, and more particularly to an optical device suitably used as an optical multiplexer or an optical demultiplexer in a system adopting wavelength division multiplexing.
2. Description of the Related Art
In recent years, processing of massive amounts of information has been needed with development of an advanced information society, and optical fiber communications fit for a large capacity have been applied to a transmission network for transmitting information. While a transmission rate of information in optical fiber communications has already reached 2.4 Gb/s or 10 Gb/s, a further increase in transmission capacity will be needed in a motion picture captured communications system that is expected to be put to practical use in the future. For example, a transmission capacity exceeding 1 terabits per second (Tb/s) will be needed in a trunk system.
Wavelength division multiplexing (WDM) is known as one of the techniques for increasing a transmission capacity in optical fiber communications. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. A plurality of optical signals obtained by independently modulating the plural optical carriers are wavelength division multiplexed by an optical multiplexer, and the resultant WDM optical signals are sent out to an optical fiber transmission line. On a receiving side, the WDM optical signals received are separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to the optical signals. Accordingly, by applying WDM, a transmission capacity in one optical fiber can be increased according to the number of WDM channels.
On the other hand, in performing WDM, the number of WDM channels is set to a wide range of several channels to about 100 channels depending upon systems. Further, a wide range of wavelength spacing of 1 nm or less to tens of nm is required. In applying WDM to a subscriber system, it is required to provide components at low prices. Accordingly, in WDM, a WDM filter usable as an optical multiplexer and/or an optical demultiplexer is a key device.
In another aspect, the application of WDM has recently been tried also in the field of measurement, and a WDM filter is an important component also in this field.
FIG. 1 is a plan view showing a conventional WDM filter usable as an optical multiplexer and/or an optical demultiplexer. This WDM filter includes a pair of slab optical waveguides (planar optical waveguides) 2 and 4 and a plurality of optical waveguides (arrayed optical waveguides) 6 for connecting the slab optical waveguides 2 and 4. The optical waveguides 6 have different optical path lengths. More specifically, the optical waveguides 6 are formed so that a phase difference by an integral multiple of 2.pi. between any adjacent ones of the optical waveguides 6 is given to light having a specific wavelength.
To obtain the function of an optical demultiplexer, at least one input optical waveguide 8 is connected to the slab optical waveguide 2 on the side opposite to the optical waveguides 6, and a plurality of output optical waveguides 10 are connected to the slab optical waveguide 4 on the side opposite to the optical waveguides 6. Diffraction occurs in a diffraction grating including the optical waveguides 6, and as the result the input optical waveguide 8 and each output optical waveguide 10 are coupled together by a specific wavelength. Accordingly, when WDM optical signals are supplied to the input optical waveguide 8, the optical signals in different wavelength channels are respectively output from the output optical waveguides 10.
In the case of using this WDM filter as an optical multiplexer, optical signals in different wavelength channels are respectively supplied to the optical waveguides 10. The optical signals are then wavelength division multiplexed, and the resultant WDM optical signals are output from the optical waveguide 8.
FIG. 2 is a plan view showing another conventional WDM filter usable as an optical multiplexer and/or an optical demultiplexer. This WDM filter includes a slab optical waveguide 12 having end faces 12A and 12B, a plurality of first optical waveguides 14 optically connected to the end face 12A of the slab optical waveguide 12, and a plurality of second optical waveguides 16 optically connected to the end face 12B of the slab optical waveguide 12. One of two end portions of each optical waveguide 16 is optically connected to the end face 12B of the slab optical waveguide 12, and a reflecting element 18 is connected directly to the other end portion of each optical waveguide 16. Each optical waveguide 16 has a substantially uniform width. To make the optical waveguides 16 and the reflecting elements 18 substantially function as a diffraction grating, the optical waveguides 16 have different optical path lengths. More specifically, the optical waveguides 1 6 are formed so that a phase different by an integral multiple of 2.pi. between any adjacent ones of the optical waveguides 16 is given to reflected light having a specific wavelength reciprocating in the optical waveguides 16. In this WDM filter, it is sufficient to provide the single slab optical waveguide 12, so that the size of the WDM filter can be made smaller than that of the WDM filter shown in FIG. 1.
In the case of using this WDM filter as an optical demultiplexer, one of the optical waveguides 14 is used as an input port, and the others are used as output ports. Conversely, in the case of using this WDM filter as an optical multiplexer, one of the optical waveguides 14 is used as an output port, and the others are used as input ports.
FIG. 3 is a plan view showing a further conventional WDM filter usable as an optical multiplexer and/or an optical demultiplexer. To reduce the size of the WDM filter shown in FIG. 1 to a substantially half, a substrate 20 is cut so that the length of each optical waveguide 6 becomes just half, and a reflecting film 22 is formed on a cut end face 20A of the substrate 20.
In the case that the WDM filter shown in FIG. 3 is used as an optical demultiplexer, one of the optical waveguides 10 is used as an input port, and the others are used as output ports. Conversely, in the case that the WDM filter is used as an optical multiplexer, one of the optical waveguides 10 is used as an output port, and the others are used as input ports.
The WDM filter shown in FIG. 1 tends to become large in size. Such a large size is due to the facts that the optical waveguides 6 must be made long to generate an optical path difference required in the diffraction grating including the optical waveguides 6 and that the two slab optical waveguides 2 and 4 are required.
The WDM filter shown in FIG. 2 has a problem that it is not easy to manufacture because a manufacturing process for each reflecting element 18 is complicated. For example, in the case of providing each reflecting element 18 by a diffraction grating, it is necessary to carry out a complicated manufacturing process including plural times of exposure for obtaining the diffraction grating.
It may be tried to obtain each reflecting element 18 by a simple process including the steps of forming an end face of each optical waveguide 16 perpendicular to its optical path and forming a reflection film directly on the perpendicular end face. Although the perpendicular end face can be obtained by etching, the etching causes a substantial deterioration of the perpendicularity of the end face, especially an edge portion of the end face. Such a deterioration of the perpendicularity is partially due to the fact that, for example, the etching rate for silica glass suitable as the material of each optical waveguide 16 is low.
To the contrary, the WDM filter shown in FIG. 3 is relatively easy to manufacture. That is, in manufacturing the WDM filter shown in FIG. 2, it is difficult to provide each reflecting element 18 by a cut end face of the substrate because the reflecting elements 18 are not arranged on a straight line. To the contrary, in manufacturing the WDM filter shown in FIG. 3, the reflecting film 22 can be easily formed because the cut end face 20A of the substrate 20 is straight and its perpendicularity is satisfactory. However, although the WDM filter shown in FIG. 3 can be reduced in size to a substantially half as compared with the WDM filter shown in FIG. 1, the size reduction of the WDM filter shown in FIG. 3 is insufficient as compared with the WDM filter shown in FIG. 2.